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The Olimpíada Brasileira de Química (OBQ) is a free, highly competitive annual examination administered by the ABQ. It serves as Brazil's official pathway to the IChO and OIAQ. This 100-question practice bank covers General & Inorganic Chemistry (25%), Olympic Physical Chemistry (25%), Organic Chemistry & Mechanisms (25%), Analytical Chemistry & Equilibria (15%), and Biochemistry & Environmental Chemistry (10%).

Sample OBQ Practice Questions

Try these sample questions to test your OBQ exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1Which set of quantum numbers (n, l, ml, ms) represents a valid electron state in an isolated atom according to the Pauli exclusion principle and quantum mechanical rules?
A.n = 2, l = 2, ml = 0, ms = -1/2
B.n = 3, l = 1, ml = -2, ms = +1/2
C.n = 4, l = 2, ml = -1, ms = +1/2
D.n = 3, l = 0, ml = 0, ms = +1
Explanation: According to quantum mechanical rules: the principal quantum number n is a positive integer (n >= 1); the angular momentum quantum number l can take integer values from 0 to n - 1 (for n = 4, l can be 0, 1, 2, or 3, corresponding to s, p, d, and f orbitals); the magnetic quantum number ml can take integer values from -l to +l (for l = 2, ml can be -2, -1, 0, +1, +2); and the spin quantum number ms must be either +1/2 or -1/2. Thus, (4, 2, -1, +1/2) represents a fully permissible 4d electron state.
2What is the ground-state electronic configuration of the gaseous iron(III) cation, Fe3+ (atomic number Z = 26)?
A.[Ar] 3d5
B.[Ar] 4s2 3d3
C.[Ar] 4s1 3d4
D.[Ar] 3d6
Explanation: Neutral iron (Z = 26) has the ground-state configuration [Ar] 4s2 3d6. When transition metal atoms ionize, electrons are first removed from the outermost valence 4s subshell before the 3d subshell due to higher radial extension and shielding. Removing three electrons removes both 4s electrons and one 3d electron, giving [Ar] 3d5, an exceptionally stable half-filled d-subshell configuration.
3According to Slater's rules, what is the effective nuclear charge (Zeff) experienced by a 4s valence electron in a ground-state neutral zinc atom (Zn, Z = 30, electronic configuration: 1s2 2s2 2p6 3s2 3p6 3d10 4s2)?
A.5.65
B.2.35
C.6.00
D.4.35
Explanation: According to Slater's rules for an s-electron in group (4s): 1) The 1 other electron in the same (4s) group contributes 1 * 0.35 = 0.35 to the shielding constant; 2) The 18 electrons in the (n-1) shell (3s2 3p6 3d10) contribute 18 * 0.85 = 15.30; 3) The 10 electrons in (n-2) and lower shells (1s2 2s2 2p6) contribute 10 * 1.00 = 10.00. Total shielding constant S = 0.35 + 15.30 + 10.00 = 25.65. Therefore, Zeff = Z - S = 30 - 25.65 = 4.35.
4An unknown main-group element X in Period 3 displays the following successive ionization energies (in kJ/mol): IE1 = 578, IE2 = 1817, IE3 = 2745, IE4 = 11578, IE5 = 14842. In which IUPAC group of the periodic table is element X located?
A.Group 2
B.Group 13
C.Group 14
D.Group 15
Explanation: A massive jump in ionization energy occurs between IE3 (2745 kJ/mol) and IE4 (11578 kJ/mol), more than quadrupling the energy required to remove the next electron. This large disparity indicates that the first three electrons are valence electrons in the outermost shell (3s2 3p1), and the fourth electron is being removed from a tightly bound noble-gas core (the 2p subshell of neon). Thus, element X possesses 3 valence electrons and belongs to Group 13 (Aluminium).
5Use the Born-Haber cycle data to calculate the lattice energy (ΔH_lattice) of calcium fluoride, CaF2(s), defined as CaF2(s) -> Ca2+(g) + 2 F-(g). Given: ΔHf°[CaF2(s)] = -1228 kJ/mol; Sublimation enthalpy of Ca(s) = +178 kJ/mol; First ionization energy of Ca = +590 kJ/mol; Second ionization energy of Ca = +1145 kJ/mol; F-F bond dissociation energy = +158 kJ/mol (for F2 -> 2 F); Electron affinity of F(g) = -328 kJ/mol.
A.+2315 kJ/mol
B.+2971 kJ/mol
C.+1987 kJ/mol
D.+2643 kJ/mol
Explanation: In a Born-Haber cycle, the formation enthalpy relates to the component steps by: ΔHf° = ΔHsub(Ca) + IE1(Ca) + IE2(Ca) + BDE(F2) + 2*EA(F) - ΔH_lattice. Substituting the values: -1228 = 178 + 590 + 1145 + 158 + 2*(-328) - ΔH_lattice. -1228 = 2071 - 656 - ΔH_lattice = 1415 - ΔH_lattice. Solving for lattice energy yields ΔH_lattice = 1415 - (-1228) = +2643 kJ/mol.
6According to VSEPR theory, what are the electron-pair geometry, molecular shape, and approximate bond angles of xenon tetrafluoride (XeF4)?
A.Octahedral electron geometry, square planar molecular shape, and 90° bond angles
B.Tetrahedral electron geometry, tetrahedral molecular shape, and 109.5° bond angles
C.Trigonal bipyramidal electron geometry, see-saw molecular shape, and 90°/120° bond angles
D.Octahedral electron geometry, square pyramidal molecular shape, and 88° bond angles
Explanation: The central xenon atom in XeF4 has 8 valence electrons and forms 4 single covalent bonds with fluorine atoms, leaving 2 non-bonding lone pairs (steric number = 4 + 2 = 6). A steric number of 6 corresponds to an octahedral electron-pair geometry. To minimize lone pair-lone pair electrostatic repulsions, the two lone pairs occupy trans-axial positions 180° apart, leaving the 4 fluorine atoms at equatorial vertices in a square planar molecular geometry with 90° F-Xe-F bond angles.
7According to molecular orbital (MO) theory for second-row homonuclear diatomic species, what is the bond order and magnetic behavior of the peroxide dianion, O2^(2-)?
A.Bond order = 2.0, paramagnetic
B.Bond order = 1.5, paramagnetic
C.Bond order = 1.0, diamagnetic
D.Bond order = 0.5, diamagnetic
Explanation: The peroxide ion O2^(2-) contains 14 valence electrons (6 from each oxygen + 2 from the negative charge). Its valence molecular orbital configuration is (σ2s)2 (σ*2s)2 (σ2pz)2 (π2px)2 (π2py)2 (π*2px)2 (π*2py)2. The number of bonding electrons is Nb = 8 and antibonding electrons is Na = 6. Bond order = (Nb - Na)/2 = (8 - 6)/2 = 1.0. Because all electrons in all occupied molecular orbitals are completely paired, O2^(2-) is diamagnetic.
8For the octahedral coordination complex ion [CoF6]^(3-), given that fluoride (F-) is a weak-field ligand and cobalt is in the +3 oxidation state (Co3+, 3d6), what is the crystal field stabilization energy (CFSE, ignoring pairing energy) and the number of unpaired electrons?
A.CFSE = -2.4 Δo, 0 unpaired electrons (low-spin)
B.CFSE = -0.4 Δo, 4 unpaired electrons (high-spin)
C.CFSE = -0.8 Δo, 2 unpaired electrons (intermediate)
D.CFSE = -1.2 Δo, 4 unpaired electrons (high-spin)
Explanation: Co3+ has a 3d6 electron configuration. Because fluoride is a weak-field ligand (low in the spectrochemical series), the crystal field splitting Δo is smaller than the spin-pairing energy P, resulting in a high-spin complex with electron configuration t2g^4 eg^2. CFSE = (4 electrons * -0.4 Δo) + (2 electrons * +0.6 Δo) = -1.6 Δo + 1.2 Δo = -0.4 Δo. There are 4 unpaired electrons (two in eg and two singly occupied in t2g), making the complex strongly paramagnetic.
9Which type of structural coordination isomerism is demonstrated by the compound pair [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4?
A.Ionization isomerism
B.Linkage isomerism
C.Coordination isomerism
D.Hydrate (solvate) isomerism
Explanation: Ionization isomerism occurs when an anionic ligand inside the first coordination sphere exchanges positions with a counter-ion in the outer ionization sphere. In [Co(NH3)5(SO4)]Br, sulfate is coordinated to Co(III) and bromide is the free outer counter-ion (precipitating AgBr with AgNO3). In [Co(NH3)5Br]SO4, bromide is coordinated to Co(III) and sulfate is the free outer counter-ion (precipitating BaSO4 with BaCl2).
10Which of the following high-spin octahedral transition metal complex ions is predicted to exhibit a strong Jahn-Teller geometric distortion in its ground electronic state?
A.[Fe(H2O)6]^(3+) (high-spin d5)
B.[Cr(H2O)6]^(3+) (d3)
C.[Cu(H2O)6]^(2+) (d9)
D.[Ni(H2O)6]^(2+) (d8)
Explanation: The Jahn-Teller theorem states that any non-linear molecular system in a degenerate electronic state will undergo geometric distortion to remove the degeneracy and lower its energy. In octahedral [Cu(H2O)6]^(2+), Cu2+ has a d9 configuration with electronic distribution t2g^6 eg^3. The eg subshell contains 3 electrons in 2 degenerate orbitals (dz2 and dx2-y2), which is an asymmetrical electronic occupancy that causes strong tetragonal elongation along the z-axis.

About the OBQ Exam

The Olimpíada Brasileira de Química (OBQ) is Brazil's premier annual chemistry competition for secondary school students, organized by the Associação Brasileira de Química (ABQ) in partnership with the Universidade Federal do Ceará (UFC). Established to discover young scientific talent, stimulate interest in chemical sciences, and select the Brazilian delegations for the International Chemistry Olympiad (IChO) and the Ibero-American Chemistry Olympiad (OIAQ), the competition features two primary streams: Modalidade A (1st and 2nd years of Ensino Médio) and Modalidade B (3rd year of Ensino Médio). Contestants qualify through their state Chemistry Olympiad (Olimpíada Estadual) and then sit a single 4-hour in-person national theoretical exam per modality, which mixes objective questions (with a −0.5-point penalty per wrong objective answer) and analytical-expository open-response problems covering advanced general, physical, inorganic, and organic chemistry; non-programmable scientific calculators are permitted. Top performers earn national medals, certificates of scientific excellence, and eligibility for Olympic university admissions (Vagas Olímpicas) at premier Brazilian research institutions such as USP, UNICAMP, UNESP, UFMG, and UFC, bypassing the traditional vestibular entrance exams. This practice bank provides 100 rigorous English-language practice items modeled after the official syllabus.

Assessment

Single national in-person theoretical exam per modality (Modalidade A: 1st/2nd-year high school; Modalidade B: 3rd-year high school), 4 hours, mixing objective questions (−0.5-point penalty per wrong objective answer) with analytical-expository open-response items. Students qualify for the OBQ through their state Chemistry Olympiad (Olimpíada Estadual), which state coordinators may run in one or two phases or via the PNOQ online Seletivas Estaduais. The separate Seletivas Internacionais stage (training course plus theoretical and practical exams) selects the IChO/OIAQ teams and is not part of the OBQ exam.

Time Limit

4 hours

Passing Score

State qualification quotas and national medal thresholds

Exam Fee

Free (Gratuito) (Associação Brasileira de Química (ABQ) / Programa Nacional Olimpíadas de Química (PNOQ))

OBQ Exam Content Outline

Not published

Química Geral e Inorgânica

Atomic structure models and quantum mechanics fundamentals; quantum numbers (n, l, ml, ms), Pauli exclusion principle, Hund's rule, and Aufbau configurations. Periodic trends, shielding effects (Slater's rules), effective nuclear charge (Zeff), ionization energies, electron affinities, and electronegativity. Chemical bonding: lattice energy, Born-Haber thermochemical cycles, VSEPR molecular geometry, hybridization, and molecular orbital theory (MO) for homonuclear/heteronuclear diatomics. Coordination chemistry: nomenclature, isomerism, Crystal Field Theory (octahedral/tetrahedral splitting, CFSE, high/low spin), Jahn-Teller effect, spectrochemical series, trans effect, organometallics (18-electron rule), and solid-state crystal lattices (BCC, FCC, density calculations).

Not published

Físico-Química Olímpica

Chemical thermodynamics: standard enthalpy changes, Hess's law, bond enthalpies, calorimetry, entropy changes (ΔS°), Gibbs free energy (ΔG° = ΔH° - TΔS°), thermodynamic spontaneity, and the van 't Hoff equation. Dynamic chemical equilibrium: Kc, Kp, reaction quotient (Q), Le Chatelier's principle, and gaseous equilibria. Chemical kinetics: differential and integrated rate laws (zero, first, and second order), reaction half-lives, Arrhenius equation (Ea determination), steady-state approximation, and catalysis. Electrochemistry: oxidation states, balancing redox reactions, standard reduction potentials, galvanic cells, Nernst equation for non-standard cells and concentration cells, Faraday's laws of electrolysis, and electrochemical corrosion mechanisms.

Not published

Química Orgânica e Mecanismos

IUPAC nomenclature, constitutional isomerism, and stereochemistry (chirality, R/S Cahn-Ingold-Prelog priority rules, enantiomers, diastereomers, meso compounds, and optical rotation). Organic reaction mechanisms: electrophilic addition to alkenes/alkynes (Markovnikov/anti-Markovnikov regiochemistry), electrophilic aromatic substitution on substituted benzenes (directing and activating effects), nucleophilic substitution (SN1 vs SN2 kinetics, stereochemistry, and solvent effects), elimination (E1 vs E2, Zaitsev vs Hofmann regioselectivity). Carbonyl chemistry: nucleophilic addition, aldol condensation, Claisen condensation, Grignard additions, Fischer esterification, Wittig reaction, Baeyer-Villiger oxidation, pinacol rearrangements, and pericyclic [4+2] Diels-Alder cycloadditions.

Not published

Química Analítica e Equilíbrio

Complex aqueous ionic equilibria: autoionization of water, pH and pOH calculations for strong and weak acids/bases, Ostwald dilution law, polyprotic acid equilibria, salt hydrolysis, buffer solutions, and Henderson-Hasselbalch equations. Solubility equilibria: solubility product constants (Ksp), molar solubility, common ion effect, selective fractional precipitation, and complex ion dissolution equilibria (Kf). Analytical titrations: strong/weak acid-base titration curves, equivalence point pH determination, acid-base indicator transition ranges, complexometric titrations with EDTA, redox titrations (permanganometry, iodometry), and spectrophotometric analysis (Beer-Lambert law and standard addition method).

Not published

Bioquímica e Química Ambiental

Structure, stereochemistry, and ionization of amino acids; isoelectric point (pI) calculations; peptide bond properties and resonance character; protein structural levels. Carbohydrates: monosaccharides, Fischer/Haworth projections, anomeric forms, mutarotation, and glycosidic linkages. Lipids: triglycerides, fatty acids, and saponification values. Enzyme kinetics: Michaelis-Menten model (Vmax, Km) and competitive/non-competitive reversible inhibition. Environmental chemistry: stratospheric ozone catalytic depletion mechanisms by chlorofluorocarbons, greenhouse gases and global warming potentials (GWP), ocean acidification carbonate equilibria, and the 12 Principles of Green Chemistry (atom economy calculations).

How to Pass the OBQ Exam

What You Need to Know

  • Passing score: State qualification quotas and national medal thresholds
  • Assessment: Single national in-person theoretical exam per modality (Modalidade A: 1st/2nd-year high school; Modalidade B: 3rd-year high school), 4 hours, mixing objective questions (−0.5-point penalty per wrong objective answer) with analytical-expository open-response items. Students qualify for the OBQ through their state Chemistry Olympiad (Olimpíada Estadual), which state coordinators may run in one or two phases or via the PNOQ online Seletivas Estaduais. The separate Seletivas Internacionais stage (training course plus theoretical and practical exams) selects the IChO/OIAQ teams and is not part of the OBQ exam.
  • Time limit: 4 hours
  • Exam fee: Free (Gratuito)

Keys to Passing

  • Work through all 100 available questions
  • Review every answer and explanation
  • Track weak areas and revisit them
  • Use our AI tutor for tough concepts

OBQ Study Tips from Top Performers

1Master Rigorous Physical Chemistry Calculations: Dedicate substantial preparation time to chemical thermodynamics (Hess's law, ΔG° = -RT ln K), chemical kinetics (integrated rate laws, Arrhenius equation), and ionic equilibria (polyprotic acids, buffer systems, Henderson-Hasselbalch, and Ksp calculations with complexation).
2Develop Fluency in Organic Reaction Mechanisms: Understand electronic movements, carbocation stabilities, nucleophilic substitution pathways (SN1 vs SN2), elimination mechanisms (E1 vs E2), electrophilic additions with Markovnikov regioselectivity, and substituent directing effects in aromatic rings (ortho/para vs meta directors).
3Strengthen Quantitative Stoichiometry & Electrochemistry Skills: Practice multi-step stoichiometric yield problems, combustion analysis for empirical formulas, gas mixtures using Dalton's and Amagat's laws, Nernst equation calculations under non-standard conditions, and quantitative Faraday electrolysis problems.
4Memorize and Apply Stereochemical Rules: Be thoroughly comfortable with Cahn-Ingold-Prelog (CIP) priority rules for assigning R/S configurations to chiral stereocenters, identifying meso compounds, calculating optical isomer numbers (2^n rule), and analyzing Fischer and Haworth projections of carbohydrates.
5Simulate Timed Exam Conditions with Past Olympiad Papers: Solve past OBQ national exams and state olympiad papers under strict timing (4 hours). Focus on clear step-by-step mathematical working, correct significant figures, dimensional analysis, and balanced chemical equations.

Frequently Asked Questions

What is the Olimpíada Brasileira de Química (OBQ) and who administers it?

The Olimpíada Brasileira de Química (OBQ) is Brazil's premier high school chemistry competition, organized annually by the Associação Brasileira de Química (ABQ) in partnership with the Universidade Federal do Ceará (UFC). It aims to stimulate scientific inquiry, improve secondary school chemistry education, and select Brazil's national teams for the International Chemistry Olympiad (IChO) and the Ibero-American Chemistry Olympiad (OIAQ).

What are the modalities and phases of the OBQ examination?

The OBQ features two competition streams based on school year: Modalidade A (students in the 1st and 2nd years of Ensino Médio) and Modalidade B (students in the 3rd year of Ensino Médio). Students qualify through their state Chemistry Olympiad and then sit one national in-person theoretical exam per modality (4 hours), which combines objective questions (a wrong objective answer incurs a −0.5-point penalty) with analytical-expository open-response items. Non-programmable scientific calculators are allowed.

How are students selected for the International Chemistry Olympiad (IChO) and OIAQ?

Top-ranked gold, silver, and bronze medalists from the national OBQ are invited to the Curso de Aprofundamento e Treinamento Intensivo (Seletiva Nacional). Through a series of advanced university-level theoretical and experimental examinations administered by the ABQ committee, the top 4 students are chosen to represent Brazil at the IChO and top 4 students represent Brazil at the OIAQ.

How much does it cost to register and compete in the OBQ?

Participation in the Olimpíada Brasileira de Química is 100% free of charge (Gratuito) across all qualifying rounds, state stages, and national phases. All school registrations, exam papers, and Olympic training activities for qualified finalists are provided without examination fees.

What core subject domains are tested on the OBQ examination?

The OBQ syllabus encompasses five core domains: Química Geral e Inorgânica (25%), Físico-Química Olímpica (25%), Química Orgânica e Mecanismos (25%), Química Analítica e Equilíbrio (15%), and Bioquímica e Química Ambiental (10%), demanding rigorous analytical problem-solving and university-level chemistry knowledge.

What academic benefits and university advantages do OBQ medalists receive?

In addition to national recognition and medal honors, OBQ medalists are eligible for direct admission without traditional entrance exams (Vestibular / ENEM) through Olympic Admission Quotas (Vagas Olímpicas) at premier Brazilian universities including USP, UNICAMP, UNESP, UFMG, and UFC in competitive programs like Medicine, Chemical Engineering, Computer Science, and Pure Chemistry.